Various processes of fabricating III-V compound semiconductor layers require the use of arsenic and phosphorous in, for example, gallium arsenide, gallium phosphide, gallium arsenide phosphide, and the like. With molecular beam epitaxy, (MBE), elemental arsenic and phosphorous are often heated to obtain the species necessary to grow the desired semiconductor layers. However, the species of arsenic, i.e., As.sub.4, derived from heating elemental arsenic or phosphorous are difficult to handle and the polymer from leads to point defects or regions of high phosphorous or arsenic concentrations in the growing layer. To avoid these problems, phosphine, Ph.sub.3, and arsine, AsH.sub.3, have been used in chemical vapor deposition, (CVD), and MBE growth processes. These materials are generally broken down into smaller molecules or components by heating the molecule above its bond breaking temperature, i.e., "cracked" into useful species of P.sub.2 and As.sub.2 by passing them through a heated zone to liberated the hydrogen as H.sub.2 gas. The process of using the gas in a heated atmosphere to break down the arsine and phosphine is generally referred to as thermal cracking.
Thermal crackers in the past have involved a tube through which the gas is passed coming into direct contact with a heating wire such as a material made from tungsten and tantalum. Although these devices are capable of cracking the gases, the process of subjecting the gases to direct contact with the heater wire permits the incorporation of impurities in the gas, both tantalum or tungsten and other impurities during the process of heating the filament to a temperature sufficiently high to crack the gas. In addition, the heating filament lifetime is shortened by coming into direct contact with the gas or gases during the heating operation.
Prior art designs of thermal cracking quartz furnaces have had difficulty in producing the desired species of arsenic from arsine AsH.sub.3. L. W. Kapitan, J. Vac Science Technology, B2(2), pp. 280-284, April-June 1984, proposed a solution to this problem by inserting a tantalum heating wire within the cracking furnace. Although this produced acceptable results, it suffers from the possible contamination of the gases due to the normal breakdown of the wire and the chemical reactions with the gases to be cracked.
As the use of III-V compound materials gains increasing economic importance in the photovoltaic and semiconductor industries, the researchers have increasingly looked toward techniques which are scalable to mass production items. VCE is a highly promising growth technique for meeting the economic needs for the high volume production of III-V compound semiconductor devices. The technique involves, among other things, the controllability of MBE with the ease of scaling of CVD while minimizing the amounts of toxic gaseous by-products and high temperature handling problems. To maintain the purity of the semiconductor layers it would be desirable to have an apparatus which is capable of thermally cracking phosphine, arsine, or organic compounds such as trimethyl or triethyl gallium. It would also be desirable to have an apparatus which is capable of cracking these gases with a minimum of contamination in the process while not exposing the heater element to the actual cracked gases which can create shortened lifetime and/or create memory problems in the VCE apparatus through absorption of the gases onto the heating filament wire.